The actor, who never received his high school diploma, will be enrolling in online courses.

Mark was growing up in Boston and getting in with the wrong crowd and just stopped going. But now that he has matured, married and had four children with wife Rhea Durham, he is ready to return to school.

"I quit during the ninth grade," Mark told David Letterman on The Late Show. "They're going to give me whatever credits I already have, which is probably like one or two."

And Mark reveals that it's not just a GED that he will be receiving. He tells Letterman, "It's an actual diploma."